Dissipative load and wattmeter for ultra high frequency electric power



Feb. 10, 1948.

E. B. MOULLIN DISSIPATIVE LOAD AND WATT METER FOR ULTRA HIGH FREQUENCY ELECTRIC P OWER aEcTmc/u. nmsuzme nsmunmr 7 IMO Pli INVENTOR ATTORNEYS cmuc rowan Feb. 10, 1948. E. B. MOULLIN DISSIPATIVE LOAD AND WATT METER FOR ULTRA HIGH FREQUENCY ELE 2 Sheets-Shegt 2 EL EC Tit/on.

Filed March 9, 1945 H \HJI'M INVEN TOR. I flallz'ol 1110a Zlz'n BY LM ATTORNEYS Patented Feb. 10, 1948 ULTRA Eric Balllol Moullin, Bro

Metropollt Company Limited, Lo

asslguor to puny of Great Britain 1945', Serial No. 581,920

Application March 9,

in Great Brita Section i, Public sec,

Patent expires 3 HIGH FREQUENCY ELECTRIC oklands, Sale, England, an-Vickers Electrical ndon, England, a comin June 11, 1943 August 8, 1946 one ll, 1963 a claims. (Ci. iii-s5) The purpose of this invention is to provide a load or known and adjustable impedance for dissipation of ultra high frequency electric power and at the same time for measuring the amount of power so dissipated. In the processes of testing and adjusting ultra high frequency generators there is often required a load resistance to simulate the aerial which said generator will ultimately suppTy. It is therefore required that said load shall have the same impedance as that oi the aerial, shall be capable of dissipatin the power without overheating and shall give a measure of the dissipation. The many practical and techneed not be indicated herein, According to the present invention the dummy load is an aerial itself which radiates power in precisely the way it would do if free in space, but is enclosed in a box of construction such that it substantially absorbs all the radiated power and yet in so doing has no reaction on the radiating aerial which is therefore, in effect, in free space. The amount of power radiated can if desired be measured by a thermometer suitably placed within the said box the construc tion of which is particularly suited to such measurements.

As set forth in earlier specification of United States patent application Serial No. 581,919, filed having a relatively high electrical resistivity, provided the said shell is spaced one quarter of a wavelength from the external metal Wall.

According to the present invention any convenlent form of aerial, such for example as a half wave aerial, is placed inside a thin tube of the aforesaid resistive material. The tube may fit fairly closely round the aerial rod or may have a larger diameter: the decision as to the diameter will depend on the number of watts to be dissi Dated in the tube. Then the aerial, sheathed in the aforesaid manner, is p'aced coaxially within a closed metal cylinder whose radius exceeds that of the sheath tube by one quarter of a wavelength. The material of the resistive sheath tube must be such that the resistance of a strip one inch wide and twelve inches long must be near 1440' ohms. Experience has shown such a material can readily be made from paper impregnated with colloidal graphite and accordingly can readily be formed into a tube. If necesmy the graphite treated paper can be wound on plating at the ends,

value.

a -Bakelite" tube for further mechanical support but for the indicating aspect of this in vention it is better it should be seli-supportina It may be noted here the correct resistivity is partially a function oi the radius of the sheath. Provided this radius is not less than dick then the correct value diliers by less than one per cent from the above: but if the radius is (k016i then it should be about "Mile ohms. The treated paper can probably dissipate at least one watt per squarelnch oi area and the radius of the tube should be chosen to suit the dissipation required. A suitable thermo-couple, or several such couples in series, can be attached to the wall of the tube, which has a conveniently small thermal capacity, and used to indicate the temperature rise. The tube can be calibrated by passing a steady current through it, suitable terminal leadsbeing attached by, say. copper for this purpose, and thus the galvanometer activated by the couples can be calibrated to read directly in watts dissipated.

If the clearance between the sheath and the external metal cylinder is not li/i then the im= pedance of they aerial will have a reactive. com ponent and a resistance less than the free space Accordingly if the diameter of the euternal cylinder is made adjustable the impedance of the load will also become adjustable and this is often convenient in practice as a means cicadjusting the load to the generator. One convenient way of providing said adjustment is,,to construct a cylinder as a diamond mesh network oi thin rods-pin jointed at their crossings, in the manner of lazy tongs: and such is capable of considerable change of radius While still remaining substantially a circular cylinder. This said lattice cylinder may be inserted within a larger metal cylinder which will afford mechanical protection,

The whole device may be contained in a sub stantialthough light metal drum, with an indicator or meter on top calibrated directly in watts. Terminals may be provided for calibration with direct current. A screened cable emerges from the wall of the cylinder, being attached to the aerial at one end and atthc other to a suitable plug for connecting to a cable leading from any generator. v

Thearrangement thus forms a self-contained dissipative load and watt meter. Whilst the principle involved in this invention is applicable at any wavelength, the apparatus cumbersome for some wavelengths greater than,

as say 3/2 metres.

For the fuller understanding of the invention it will now be described by way 01 example with reference to the accompanying drawings, which are purely diagrammatic and not to scale.

Fig. l is a diagrammatic view one embodiment of the present invention.

Fig. 2 is an axial section, partly in elevation. of another embodiment of the invention.

Referring to Fig. 1 of the drawing, at i is shown an ordinary half-wave aerial rod with lines 2. At 3 is shown the tube of resistive material axially in which is located the aerial l. The tube 3 is shown as transparent in order that the aerial may be seen. At l are shown the copper-plated ends of the resistive tube 3 for the connection to the tube of terminal leads la.

At is shown the external metallic cylinder having closed ends.

The leads 4a pass through the ends of the metal cylinder 5 to terminals 6 to permit calibration.

At 1 is shown an electrical measuring instrument such as a galvanometer or fractional ammeter connected by leads la to a thermo-couple 8 on the wall of the tube 3.

The radial distance between tube 3 and the cylinder 51s one-quarter of a wavelength as indicated by-M4. The arrangement shown inthe drawing is otherwise constructed or may be modifled and is adapted to be used as hereinbefore set forth.

Fig. 2 illustrates an embodiment of the invention wherein the diameter of the external or electrically conductive cylinder is adjustable. In this modified arrangement, the external cylinder 5 of Fig. l is replaced by a lattice cylinder 9 located within a fixed box iii, the lattice cylinder being composed of thin rods pin-Jointed or pivotally connected at their intersections and providing a diamond mesh network. Adjustment of the lattice cylinder is effected by rotating a knob ll threaded on a central rod I! which is splined box i0 and is moved vertically assaid knob is rotated. Control rods I3 are connected by pin-joints to the lower end of the rod i2 and to the upper edge of the lattice cylinder, and similar rods l4 connect the lower edge of the lattice cylinder with a member i5 which is fixed in the bottom of the fixed box II). By this construction, upward movement of the rod l2 will contract the lattice cylinder and thereby reduce its diameter and the spacing between it and the resistive cylinder, and downward movement of the rod l2 will cause the lattice cylinder to expand in diameter and thereby increase its diameter and said spacing. The lattice cylinder, because of its lazy tongs" structure, will maintain its cylindrical form during its contraction and expansion.

In some applications the device according to the invention may be needed only as a dissipative load, and not simultaneously as a means of measuring the power dissipated in it. In this case the thermo-couple system can if desired be omitted, or alternatively only a single thermo-couple retained for possible use as desired as a rough indicator during preliminary adjustment. In this case, furthermore, there is no need to keep small the heat capacity of the dissipating tube 3: thus the carbon or other resistive material can be painted on a relatively more robust tube such as of the material known in the trade under the registered British trade-mark Bakelite, or embedded on a tube of the material known in the trade under the registered British trade-mark 1 supplied with 4 "Traflolyte. Thus not only can a much more robust mechanical construction be obtained, but

the resistive properties oi the tube remain appreciably more constant than is possible with a thin paper tube.

On the other hand when the device is required to act as its own watt meter measuring the power by the above described method, it is highly desirable that the heat capacity 01 the graphite cylinder shall be as small as is reasonably possible in order that the time lag of thermal response shall be desirably small. In this case in actual practice the tube has been made or a single sheet of graphite-loaded paper such as is readily obtainable on the market. Experience has shown that the ohmic resistance of such a tube tends to increase in damp weather, and accordingly it may be desirable to protect it from moisture; whilst most insulating varnishes have been found impracticable for protecting the tube against moisture owing to the fact that they wet the paper, causing its fibres to expand, with deterioration of the graphitic fllm. However, completely satisfactory results have been obtained by soaking the graphite paper tube in molten paraiiln wax, which material does not wet" the paper, and such tubes remain permanent indefinitely. Waxes of high melting point may bepreierred in order to increase the permissible temperature.

The device has been operated in conjunction with a standing wave detector in the cable 2 feeding the aerial I, said detector serving to indicate the degree or match between the load and the cable. Experience shows that the match can be adjusted conveniently by varying the diameter of the external metal cylinder such as by giving it the construction hereinbeiore set forth. Analysis has shown that the adjustment for match should be conveniently blunt, as distinct from sharp, when the clearance is of the order of one-quarter wavelength, and this is borne out in actual practice.

Experience has shown that the load device according to the invention is practicable and workable. The invention is not limited to any particular use, since it may be employed in various applications.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Apparatus for measuring ultra high frequency power, comprising an aerial adapted to be supplied with the power to electrically resistive cylindrical sheath surrounding said aerial, an electrically conductive cylinder enclosing said resistive sheath and radially spaced therefrom approximately M4, and means for measuring the heating eflect produced in said resistive sheath.

2. Apparatus for measuring ultra high irequency power, comprising an aerial adapted to be supplied with the power to be measured, an electrically resistive cylindrical sheath surrounding said aerial, an electrically conductive cylinder enclosing said resistive sheath and radially spaced therefrom approximately a quarter the wavelength of the energy to for forming a thermo couple on the surface of the resistive sheath, an strument located externally oi the conductive cylinder, and electrical leads connecting the thermo couple and the electrical measuring instrument.

3. Apparatus for measuring ultra high irequency power, comprising an aerial adapted to be the power to be measured. an elecbe measured, an

. 5 6 trically resistive cylindrical sheath surrounding enclosing said resistive sheath, means for varying said aerial, said resistive sheath having a radius the di met i n ve y n er 0 88 t at least 0.1: and being such that the resistance e fect a spaci of the co ct ve cy i der from of a strip one inch wide and twelve inches long the resistive sheath of A/4, and means for measis of the order of 14401 ohms, an electrically conuring t e hea in e e Produced in S sistive ductive cylinder enclosing said resistive sheath h athand spaced therefrom approximately v4, and ppa atus for dissipating ultra high iremeans for measuring the heating efiect produced quency powe c mprising an aerial adapted to be in said resistive sh ath, supplied with power, an electrically resistive 4, Apparatus for measuring mm; high frei0 sheath surrounding said aerial, an electrically quency power, comprising an aerial adapted to onductive cylinder surrounding said resistive be supplied with the power to be measured, an Sheath, a means for a j ing he diametrical than 0.1x and a resistance such that a strip one sheath of V inch wide and twelve inches long has a resistance ERIC BAILFOL of the order of 74011' ohms, an electrically conductive cylinder enclosing said resistive sheath REFERENCES CITED n r i y ac h f m appro imately V4, The following references are 01' record in the and means for measuring the heating eflect profile 01 this patent: duced in said resistive sheath.

5. Apparatus for measuring ultra high i're- UNITED STATES PAW quency power, comprising an aerial adapted to Number Name Date be supplied with the power to be measured, an 2,245,669 Hollmann June 17, 1941 electrically resistive cylindrical sheath surround- 2,284,379 Dow May 26, 1942 ing said aerial, an electrically conductive cylinder 1,921,117 Darboard Aug. 8, 1933 

